EP4062472A1

EP4062472A1 - Gas diffusion layer for fuel cells

Info

Publication number: EP4062472A1
Application number: EP20803494.2A
Authority: EP
Inventors: Achim Bock; Kristof Klein; Christoph Rakousky; Hannes BARSCH
Original assignee: Carl Freudenberg KG
Current assignee: Carl Freudenberg KG
Priority date: 2019-11-20
Filing date: 2020-11-05
Publication date: 2022-09-28
Anticipated expiration: 2040-11-05
Also published as: DE102019131343A1; KR20220065000A; EP4303968A3; TW202121722A; EP4062472B1; JP2023502262A; CN114641876A; TWI794685B; WO2021099129A1; DK4062472T3; US20220393184A1; EP4303968A2; CA3160120A1

Abstract

The invention relates to a gas diffusion layer for a fuel cell, comprising a) a flat, electrically conductive fiber material and b) a microporous layer on one of the surfaces of the fiber material, wherein the gas diffusion layer has, with respect to its base area (on the x-, y-plane), at least one property gradient relating to at least one chemical and/or physical property. The invention also relates to a method for producing such a gas diffusion layer, to a fuel cell which contains said diffusion layer, and to the use of the diffusion layer.

Description

Gas diffusion layer for fuel cells

BACKGROUND OF THE INVENTION

The present invention relates to a gas diffusion layer for fuel cells, fuel cells containing them and the use of such a gas diffusion layer.

Fuel cells use the chemical conversion of a fuel, in particular hydrogen, with oxygen to form water in order to generate electrical energy. In hydrogen-oxygen fuel cells, hydrogen or a hydrogen-containing gas mixture is fed to the anode, where electrochemical oxidation takes place with the release of electrons (H. 2 H ⁺ + 2 e). Via a membrane, which the

If reaction spaces are separated from one another in a gas-tight manner and electrically insulated, the protons are transported from the anode space into the cathode space. The electrons provided at the anode are fed to the cathode via an external conductor circuit. Oxygen or an oxygen-containing gas mixture is fed to the cathode, with a reduction of the oxygen taking place with the absorption of the electrons. The oxygen anions formed react with the protons transported across the membrane to form water (1/2 O + 2 H ⁺ + 2 e ^ For many applications, especially in the automotive drive train, low-temperature proton exchange membrane fuel cells (PEMFC, proton exchange membrane fuel cells, also known as polymer electrolyte membrane fuel cells) are used, which achieve efficiencies of up to 60%. The core of the PEMFC is a polymer electrolyte membrane (PEM) which is only ^{permeable to protons (or oxonium ions FhO +} ) and water and which spatially separates the oxidizing agent, generally atmospheric oxygen, from the reducing agent. In order to ensure the best possible proton transport across the membrane, humidified membranes are usually used. A catalyst layer is applied to the gas-tight, electrically insulating, proton-conducting membrane on the anode and cathode side, which forms the electrodes and which usually contains platinum as the catalytically active metal. The actual redox reactions and charge separations take place in the catalyst layers. Special requirements are placed on them because they must be electron and proton conductive at the same time and must also allow the supply of the reaction gases and discharge of the water formed during the cathode reaction. The membrane and catalyst layers form a unit, which is also referred to as a CCM (catalyst coated membrane). On both sides of the CCM there is a gas diffusion layer (GDL) that stabilizes the cell structure and takes on the transport and distribution functions for reaction gases, water, heat and electricity. The membrane, electrodes and gas diffusion layer form the membrane electrode assembly (MEA).

An alternative to the CCM is the gas diffusion electrode (GDE), whereby the electrode material is not applied to the membrane but to the GDL. The EP 2228857 A1 describes a membrane electrode unit for high-temperature fuel cells which comprises a gas diffusion electrode which has at least two gas diffusion layers which contain polytetrafluoroethylene, the gas diffusion layers having different polytetrafluoroethylene concentrations. The GDE thus has a PTFE concentration gradient in

Direction of passage of the operating media, d. H. with respect to their thickness or perpendicular to the base (i.e. in the direction of the z-axis).

In single cells, which only have a single membrane-electrode unit, a flow distributor plate is arranged on each side of the MEA, which has channels for supplying the process gases to the electrodes and, as a rule, additional internal cooling channels for dissipating the heat. However, fuel cells generally do not consist of individual cells, but of a large number of membrane-electrode units arranged in a stack, which are connected in series and whose electrical powers add up. Between two

Membrane-electrode units then usually only one flow distributor plate (a so-called bipolar plate) is arranged. This bipolar plate is structured on the front side and on the rear side and each has channels for supplying the cathode adjoining on one side and the anode adjoining on the other side with process gases and, as a rule, additional internal cooling channels. The flow distributor plates, or bipolar plates, consist of an electrically conductive material to establish the electrical connection between the stacked

Manufacture membrane-electrode assemblies. The main task of the flow distributor plates is the uniform supply of the MEA with reaction gases and the removal of the reaction products, ie in the case of the hydrogen-oxygen fuel cell the water formed during the cathode reaction. For this purpose, the flow distributor plates have a channel structure that is open on one side between the inlet and outlet, the so-called flow field. The flow field is used for the macroscopic distribution of the reaction gases to the adjacent GDL, which takes over the microscopic distribution to the catalytically active areas of the membrane. The flow field also serves to transport the gaseous and liquid product water away. The flow field is formed by bars and channels, the arrangement of which forms a characteristic design. Usual web widths and channel widths are each in a range of about 0.2 to 1.5 mm. The PEMFC currently uses four main flow field designs: parallel flow field with straight channels, meander flow field, interdigitated flow field with interrupted channels and the post flow field. In addition to the process gas supply, constant condensate discharge is an important criterion when designing the flowfield. Convective mass transport dominates within the flow field due to the pressure difference between the inflow and outflow of the flow field. The anode and cathode differ in their requirements with regard to the supply of process gases and the discharge of reaction products. On the anode side, the supplied hydrogen is essentially converted into protons, which migrate through the membrane to the cathode. No process water is formed during the anode reaction, but water can get from the cathode through the membrane to the anode. On the cathode side, the reduction reaction forms significant amounts of water that have to be discharged via the channel structure of the flow distributor plates. The flow distribution plates for the cathode and the The anode, or the two sides of a bipolar plate, therefore usually have different flow fields.

Between the flow distributor plates or the bipolar plates and the catalyst layers there are gas diffusion layers, which are of major importance for the function and performance of the fuel cell:

All process components consumed and created in the electrode reactions must be transported through the gas diffusion layer and homogeneously distributed from the macroscopic structure of the flow distributor plates / bipolar plates to the microscopic structure of the catalyst layers. This includes the transport of hydrogen to the anode and oxygen to the cathode as well as gaseous and liquid water from the anode into the flow channels of the flow distributor plates / bipolar plates. On the one hand, the gas transport must not be hindered by excessive flooding of the pores and, on the other hand, the MEA must not dry out.

The electrons formed and used up in the half-cell reactions must be conducted to the flow distributor plates with as little voltage loss as possible. This is achieved through the use of highly conductive materials.

The heat generated during the reaction must be dissipated to the coolant in the flow distributor plates, so that the materials of the GDL must also have sufficient thermal conductivity.

In addition, the GDL must also act as a mechanical compensation between the macrostructured flow distributor plate and the catalyst layers Act. To do this, component tolerances must be compensated and the compression pressure distributed. It also serves as mechanical protection for the very thin membranes that are exposed to high loads in the fuel cells.

Due to the required properties, GDL typically consist of a carbon fiber substrate, which is usually made hydrophobic with fluoropolymers (e.g. PTFE). In order to optimize the transport properties of gas and liquid water, the GDL are generally coated over a large area with a microporous layer (MPL). The MPL usually consists of a fluorine-containing polymer as a binder (e.g. PTFE) and a porous and electrically conductive carbon material (e.g. carbon black or graphite powder). While the pores in the fiber substrate of the GDL usually have a diameter of 10 to 30 mΐti, the pore diameter in the MPL is usually in a range from 0.05 to 1 mΐti. Since the pore size of the electrode is in the range from 10 to 100 nm, the MPL creates a transition from the macropores of the substrate to the micropores of the electrode. The following three materials are currently used as carbon fiber substrates for the GDL: B. Yarns made of oxidized but not yet carbonized polyacrylonitrile fibers are used, which are carbonized or graphitized after weaving.

Carbon fiber papers: For production, z. B. PAN fibers carbonized, shredded into fiber fragments, dissolved and carried out analogously to paper production Sieben (laid paper) made a fiber scrim. To stabilize the paper, binders, e.g. B. phenolic resins are used.

Carbon fiber nonwovens: Dry laid, carded and hydroentangled nonwovens made of oxidized polyacrylonitrile can be used for production, which are then calibrated for thickness and carbonized. A conductive nonwoven fabric and its manufacture are e.g. B. described in WO 02/31841 A2.

The GDL known from the prior art consist of layers and substructures that are as homogeneous as possible. Neither the fiber structures nor the hydrophobic finish and the MPL coating have so far had any specifically generated property gradients.

Another problem in the operation of fuel cells is that there are large differences in the chemical composition of the flowing operating media (oxygen content, hydrogen content, total water content) and the operating parameters between the supply of process gases to the flow field of the flow distributor plate and the removal of the reaction products from the flow field (Temperature, pressure, content of gaseous and liquid water) occur. Many of these parameters have a gradient both in relation to the specific flow course specified by the flowfield design and in relation to the direct connection between the supply and discharge of the operating materials. These gradients continue in the GDL up to the electrodes. They have a major influence on the performance of the fuel cell, as they lead to a gradient in the current density distribution between the input and output of the respective electrode. Above all, the gradients on the are decisive for performance Cathode side (air side) of a fuel cell, whereby gradients in the hydrogen distribution as well as passage of water through the membrane and a gradient formation associated therewith can also occur on the anode side. This gradient formation is explained below for the situation at the cathode:

Cathode input:

0 ₂ concentration = maximum 0 ₂ pressure = maximum temperature = minimum moisture content gas = minimum

Cumulative amount of liquid water = minimal

Cathode output:

0 ₂ concentration = minimum 0 ₂ pressure = minimum temperature = maximum moisture content gas = maximum amount of liquid water accumulated = maximum Attempts have already been made to reduce the formation of such gradients when the fuel cell is operating. One approach lies in the design of the flow distributor plates / bipolar plates. An attempt was made to minimize the gradient formation through the design of the flow field (e.g. cross-flow vs. counter-flow of the anode and cathode gases). WO 2015150533 describes a bipolar plate for fuel cells, the hydraulic cross-section of which is optimized in such a way that the pressure loss of the operating media is reduced and the pressure distribution of the operating media over the surface is as homogeneous as possible. A bipolar plate is used for this, which is divided into three areas, comprising two distributor areas and one active area. A first distribution area is used to supply operating media to the active area of the bipolar plate, and a second distribution area is used to discharge the operating media from the active area. In addition, the bipolar plate has channels which connect the main operating equipment ports of both distributor areas with one another. Furthermore, the distributor areas have at least one intersection section in which the channels do not intersect in a fluid-connecting manner. The cathode gas main port is arranged in such a way that cathode channels proceeding from it run in a straight line over at least the distributor area of the fuel cell, and that in a first intersection section anode channels and cathode channels proceeding from the anode gas main port intersect and enclose an angle that is greater than 0 ° and less than 90 ° is. The fuel cells described in WO 2015150533 are also in need of improvement with regard to their properties.

DE 102005022484 A1 describes a gas diffusion layer which has at least two functional areas that are operatively connected to one another, the first area having a porous structure and the second area being designed as a stabilization zone. It is described quite generally and without the support of an exemplary embodiment that the GDL can have a progressive structure, for example in the form of a gradient. For example, the GDL can consist of a consist of uniform material, which is characterized in terms of its flexural strength, its tensile modulus or other unspecified mechanical properties by gradients in different spatial directions. In particular, this document does not describe the fact that the GDL is coated with an MPL which has a continuous or abrupt property gradient in the xy plane and this gradient changes monotonically as a function of the location (ie either always increases or always decreases, whereby it can also remain constant in partial areas but has no local minima or maxima).

US 2010/0255407 A1 describes an electrode for a fuel cell which comprises a gas diffusion layer, a catalyst layer and a water-repellent material at the interface between gas diffusion layer and catalyst layer. Specifically, it is a special version of a PEM fuel cell with a phosphoric acid-soaked electrolyte membrane. The water-repellent material is intended to prevent the phosphoric acid from blocking an even flow of oxygen into the catalyst layer. It has a continuous concentration gradient in a first direction, which extends away from the gas diffusion layer (ie in the z-direction) and a discontinuous concentration gradient in a second direction, perpendicular to the first direction (ie in the x, y-plane). The water-repellent material between GDL and catalyst layer is an essential feature of the electrode described, although this does not correspond to the MPL, which can also be present. It is also of critical importance that the water-repellent material has a discontinuous gradient in the direction of the surface at the interface between GDL and catalyst layer. For this purpose, the water-repellent material can, for example, be placed on the GDL be arranged, wherein the points have a radial concentration gradient in the direction of the surface. This document also does not describe coating a GDL with an MPL which has a continuous or abrupt, monotonous property gradient in the x, y plane.

CN 110112425 describes a PEM fuel cell with a gas diffusion layer which has a hydrophobization of the fiber material along the flow channels with a gradient in the main gas flow direction between inlet and outlet. PTFE is preferably used for water repellency. The fiber material itself is hydrophobized. This document does not describe the use of a GDL which has an MPL as an additional layer which has a monotonous property gradient in the x, y plane.

There is currently a need for PEM fuel cells which are improved with regard to the complex property profile described. The present invention is based on the object of reducing or avoiding the disadvantages that result from the gradients with regard to the chemical composition and / or the operating parameters between the supply and discharge of the operating media. The fuel cells provided should be particularly characterized by the smallest possible fluctuations in the current density over the active surface.

Surprisingly, it has now been found that this object is achieved by a gas diffusion layer for fuel cells, which in turn has at least one property gradient, as a result of which the distribution of the operating media through the GDL is improved. By specifically adapting the GDL properties to the Gradients of the operating media, as specified by the ambient conditions of the half-cell reactions, can significantly increase the performance of the fuel cell. In particular, the fluctuations in the current density over the active area can be reduced.

SUMMARY OF THE INVENTION

A first object of the invention is a gas diffusion layer for a fuel cell, comprising a) a flat, electrically conductive fiber material and b) a microporous layer on one of the surfaces of the fiber material, the gas diffusion layer in relation to its base area (in the x, y plane) at least has a property gradient with respect to at least one chemical and / or physical property.

In a preferred embodiment, at least the microporous layer has at least one property gradient.

Another object of the invention is a method for producing a gas diffusion layer for a fuel cell, which comprises a flat, electrically conductive fiber material a) and a microporous layer b) on one of the surfaces of the fiber material, the microporous layer in relation to the base area of the Gas diffusion layer (in the x, y plane) has at least one property gradient with respect to at least one chemical and / or physical property, in which i) a flat, electrically conductive fiber material a) is provided, ii) the fiber material provided in step i) with a precursor coated to form a microporous layer, the composition of the precursor being varied during the coating to produce a gradient, iii) subjecting the coated fiber material obtained in step ii) to a thermal aftertreatment.

Another subject matter of the invention is a fuel cell which comprises at least one gas diffusion layer, as defined above and below.

Another object of the invention is the use of a gas diffusion layer, as defined above and below, for the production of fuel cells with reduced fluctuations in the current density over the electrode surfaces, especially on the cathode side.

DESCRIPTION OF THE INVENTION

The fuel cells according to the invention have the following advantages: Due to the at least one property gradient that the gas diffusion layers according to the invention have, the properties of the GDL can be specifically adapted to the operating conditions of the respective fuel half cell. The GDL is characterized by improved properties with regard to the distribution of the operating materials. In particular, the GDL manages to control various transport processes independently of one another. For example, the transport of liquid water and gaseous water can be set separately. The oxygen transport through the GDL to the cathode can also be controlled in a targeted manner.

The gas diffusion layers according to the invention can be produced simply and inexpensively.

By using the gas diffusion layers according to the invention, fluctuations in the current density over the active surface can be reduced in the resulting fuel cells.

Gas diffusion layer (GDL)

The GDL used according to the invention is a sheet-like structure which has an essentially two-dimensional, planar extent and, in contrast, a smaller thickness. The gas diffusion layer according to the invention has a base area which as a rule essentially corresponds to the base area of the adjoining membrane with the catalyst layers and the base area of the adjoining flow distributor plate. The shape of the base area of the gas diffusion layer can, for example, be polygonal (n-angular with n> 3, e.g. triangular, square, pentagonal, hexagonal, etc.), circular, circular segment-shaped (e.g. semicircular), elliptical or elliptical segment-shaped. The base area is preferably rectangular or circular. In the context of the invention, an orthogonal coordinate system is used to describe the GDL, the base area of the GDL lying in the plane spanned by the x-axis and the y-axis (also referred to as the x, y-plane). The orthogonal z-axis is used to describe the material thickness. According to the usual description for fiber composite materials, the x-axis is also described as the roll direction (machine direction, MD) and the y-axis as the counter-roll direction (cross machine direction, CMD). In the direction of the z-axis, the substance is essentially transported between the flow distributor plate and the membrane.

According to the invention, the gas diffusion layer has at least one property gradient with regard to at least one chemical and / or physical property. That is to say, at least one property of the gas diffusion layer depends on the location. The property gradient can extend over one, two or all three spatial directions. It can each extend over the full length extension in one spatial direction or over a specific section. The change in properties can be abrupt (i.e. the gas diffusion layer according to the invention has a heterogeneity with respect to at least one property) or it can be continuous (i.e. that according to the invention

Gas diffusion layer has an inhomogeneity with regard to at least one property). A sudden change in properties generally has at least 2, preferably at least 3, in particular at least 4 steps with regard to the property exhibiting the gradient. Both the flat fiber material a), as the microporous layer b), as well as both, can have the at least one property gradient.

At least the microporous layer preferably has at least one property gradient. At least the cathode-side gas diffusion layers of the fuel cells according to the invention preferably have an MPL which has a property gradient in relation to the base area (in the x, y plane) of the GDL. It has been found that by using an MPL which has a property gradient in relation to the base area (in the x, y plane) of the GDL, a more uniform current density distribution of the fuel cell can be achieved. In a special embodiment, only the microporous layer has one or more property gradients.

The gas diffusion layer (i.e. the flat fiber material a) and / or the microporous layer b)) preferably has at least one property gradient which is

Monotonically changes depending on the location. A monotonic change in property is understood to mean that the function value which represents the change in property either always increases or always falls when the value for the location coordinate increases. It is permitted that the function value, which represents the change in properties, remains the same in the course of the location coordinate even over a sub-area or over several sub-areas. However, he has no local

Minima or maxima. The gas diffusion layer (ie only the flat fiber material a) or only the microporous layer b) or the flat fiber material a) and the microporous layer b)) preferably only has property gradients which change monotonically depending on the location.

At least the microporous layer b) preferably has at least one property gradient which changes monotonically as a function of the location. In a special embodiment, only the microporous layer has at least one property gradient which changes monotonically as a function of the location. In a further special embodiment, the microporous layer only has property gradients which change monotonically as a function of the location. More specifically, the microporous layer has only a single property gradient and this one property gradient changes monotonically as a function of the location.

The gas diffusion layer comprises as component a) at least one electrically conductive flat fiber material. Component a) preferably comprises a fiber material which is selected from nonwovens, papers, woven fabrics and combinations thereof.

Suitable substrate materials are fiber materials which are themselves conductive or which are made conductive by adding conductive additives such as carbon or metal particles. In principle, carbon fibers, glass fibers, fibers of organic polymers such as polypropylene, polyester, polyphenylene sulfide, polyether ketones and mixtures thereof are suitable as substrate material. The fibers contained in the fiber material a) comprise or consist preferably of carbon fibers (carbon fibers, carbon fibers). Such fiber materials particularly advantageously meet the requirements placed on the GDL for gas diffusivity, liquid water permeability, electrical and thermal conductivity. The carbon fibers can be produced in the usual way, with polyacrylonitrile fibers (PAN fibers) preferably being used as the starting material. PAN fibers are produced by radical polymerization of a monomer composition which preferably contains at least 90% by weight, based on the total weight of the monomers used for the polymerization, of acrylonitrile. The polymer solution obtained is, for. B. by wet spinning and coagulation, spun into filaments and combined into ropes. Before this PAN precursor is converted to carbon fibers at high temperatures, it is usually subjected to an oxidative cyclization (also referred to as oxidation) in an oxygen-containing atmosphere at elevated temperatures of around 180 to 300 ° C. The resulting chemical crosslinking improves the dimensional stability of the fibers. The actual pyrolysis to carbon fibers then takes place at temperatures of at least 1200 ° C. For this pyrolysis, depending on the shape of the desired fiber material, either the starting fibers or a flat fiber material can be used. Depending on the temperature during pyrolysis, a distinction is made between carbonization and graphitization. Carbonization refers to a treatment at around 1200 to 1500 ° C under an inert gas atmosphere, which leads to the elimination of volatile products. So-called high-modulus or graphite fibers are obtained through graphitization, ie heating to around 2000 to 3000 ° C under inert gas. These fibers have a high degree of purity, are light, very strong and very good conductive for electricity and heat.

The fiber material a) is preferably selected from carbon fiber fabrics, carbon fiber papers and carbon fiber nonwovens. In the case of carbon fiber fabrics, the flat fiber material is produced by crossing two thread systems, warp (warp threads) and weft (weft threads). As with textiles, fiber bundles are flexibly but inextricably linked. For the production of carbon fiber fabrics, oxidized, but not yet carbonized or graphitized PAN fibers are preferably used. The carbonization or graphitization, in order to give the flat fiber material electrical conductivity, takes place after weaving.

As described at the beginning, oxidized PAN fibers are generally used to produce carbon fiber papers. These are comminuted into fiber fragments in a manner known per se, slurried and, analogously to paper production, a fiber scrim is produced by sieving (laid paper) and dried. In a preferred embodiment, at least one binding agent is additionally introduced into the paper. Suitable binders are e.g. B. phenolic, furan, polyimide resins, etc. To introduce the binder, the paper can be impregnated with this and the binder can then be hardened if necessary. After impregnation and hardening, the carbon fiber paper is once again subjected to carbonization / graphitization in order to convert the binder into compounds with improved electrical conductivity. In a further suitable embodiment, a filled carbon fiber paper is used to provide the fiber material a). Fiering is initially carried out as described above, but instead of introducing a binding agent and carbonizing / graphitizing a filler made of a carbon material in a polymeric binder, the still moist paper is introduced. A carbon-PTFE filler is used specifically for this purpose. Through this Filling, the thermal and electrical conductivity is increased so that carbonization / graphitization can be omitted.

Non-oxidized or oxidized PAN fibers can be used to manufacture carbon fiber nonwovens. In a first step, these can be laid dry to form a pile (carded) and then consolidated into a fleece. This can be done, for example, by water-jet entangling (hydro-entangling), with the carbon fibers being oriented, interlaced and thus mechanically stabilized. If necessary, the thickness of the bonded nonwoven fabric can be calibrated to a desired value. Nonwovens based on non-oxidized PAN fibers are first subjected to oxidation at elevated temperature and under an oxygen atmosphere and then to carbonization / graphitization under an inert gas atmosphere after the nonwoven fabric has been laid and solidified. Nonwovens based on oxidized PAN fibers are only subjected to carbonization / graphitization after the laying and consolidation of the nonwoven. Optionally, at least one binding agent can additionally be introduced into the fleece and this can then be hardened if necessary. Suitable binders are those mentioned for carbon fiber papers, especially phenolic resins. The introduction of the binder can be, for. B. connect to the carbonization / graphitization and the impregnated fleece obtained are finally carbonized / graphitized again.

In a special embodiment, the flat, electrically conductive fiber material a) comprises at least one carbon fiber nonwoven. These are advantageous, among other things, because they are compression-elastic and easy to use on a large scale, e.g. B. in a roll-to-roll process. The fiber material a) is generally a fiber composite material comprising: a1) carbon fibers, a2) optionally at least one polymeric binder and / or a pyrolysis product thereof, a3) optionally at least one further additive different from a2).

The fiber materials a) contained in the gas diffusion layer can contain conventional additives a3). These are preferably selected from flydrophobizing agents, conductivity-improving additives, surface-active substances and mixtures thereof.

In order to improve the transport processes through the GDL and at the interfaces, it can be advantageous to increase the flydrophobicity of the fiber material a). Suitable hydrophobizing agents are fluorine-containing polymers such as polytetrafluoroethylene (PTFE) and tetrafluoroethylene-hexafluoropropylene copolymers (FEP). The hydrophobizing agent used is preferably PTFE. The fiber material can be finished with the water repellent by customary impregnation processes. For this purpose, a PTFE dispersion can be applied in an immersion bath, the solvent evaporated and the treated fiber material sintered at elevated temperatures of generally at least 300.degree. The fiber material a) preferably has a content of water repellants of 3 to 40% by weight, based on the total weight of the fiber material a). In a special embodiment, the fiber material has a PTFE content of 3 to 40% by weight, based on the total weight of the fiber material a).

To improve the electrical and thermal conductivity, the fiber material a) can be equipped with at least one conductivity-improving additive. Suitable conductivity-improving additives are, for. B. metal particles, carbon particles, etc. The conductivity-improving additive is preferably selected from carbon black, graphite, graphene, carbon nanotubes (CNT), carbon nanofibers and mixtures thereof. The fiber material a) can be provided with at least one conductivity-improving additive, for example, together with the water repellent, especially a PTFE dispersion. In many cases, the fiber material a) has good electrical and thermal conductivity due to the carbon fibers used, even without conductivity-improving additives.

The fiber material a) preferably has a content of conductivity-improving additives of 0 to 40% by weight, based on the total weight of the fiber material a). If the fiber material a) contains a conductivity-improving additive, then preferably in an amount from 0.1 to 40% by weight, particularly preferably from 0.5 to 30% by weight, based on the total weight of the fiber material a).

The fiber material a) preferably has a thickness in the range from 50 to 500 μm, particularly preferably from 100 to 400 μm. This thickness refers to the uncompressed state of the fiber material a), ie before the GDL is installed in a fuel cell.

The fiber material a) preferably has a porosity in the range from 10 to 90%, particularly preferably from 20 to 85%. If the fiber density is known, the porosity of the fiber material can be calculated from the measured thickness and the measured weight per unit area. For a carbon fiber ^{density of 1.8 g / cm 3} : porosity [%] = [(1.8 weight per unit area / thickness) / 1.8] x 100. It is also possible to determine the density of the gas diffusion layer per Flelium -Density measurement and the specific pore volume to be determined using mercury porosimetry. The porosity is then calculated as follows: Porosity [%] = specific pore volume / (specific pore volume + 1 / fleece density) x 100%.]

The mean pore diameter of the fiber material a) is preferably in a range from 5 to 60 μm, particularly preferably from 8 to 50 μm, in particular from 10 to 40 μm. The mean pore diameter can be determined using mercury porosimetry.

The gas diffusion layer according to the invention consists of a two-layer composite layer based on a flat, electrically conductive fiber material a) and a microporous layer (MPL) b) on one of the surfaces of the fiber material a).

In contrast to the macroporous fiber material a), the MPL is microporous with pore diameters that are generally well below one micrometer, preferably of at most 900 nm, particularly preferably at most 500 nm, in particular of at most 300 nm. The mean pore diameter of the M PL b) is preferably in a range from 5 to 200 nm, particularly preferably from 10 to 100 nm. The mean pore diameter can in turn be determined by mercury porosimetry. The mean pore diameters mentioned last apply above all to the use of carbon black as conductive particles in the MPL. By using graphite as conductive particles in the MPL or the use of pore formers, significantly larger MPL pores can also be created. Depending on the composition, the mean pore diameter is then z. B. greater than 1 pm. When using different conductive particles, the pore diameter can have a bimodal or polymodal distribution curve. When using a mixture of carbon black and graphite, a distribution of the pore diameter with two pore peaks (one carbon black and one graphite peak) can be obtained.

The MPL contains conductive carbon particles, preferably carbon black or graphite, in a matrix made of a polymeric binder. Preferred binders are the aforementioned fluorine-containing polymers, especially polytetrafluoroethylene (PTFE).

The microporous layer b) preferably has a thickness in the range from 10 to 100 μm, particularly preferably from 20 to 50 μm. This thickness relates to the uncompressed state of the microporous layer b), i. H. before installing the GDL in a fuel cell.

The presence of the MPL has a major influence on the water balance of the fuel cell. Due to the high PTFE content and the smaller pores of the MPL, the flooding of the GDL and the electrode is made more difficult because the MPL as Liquid water barrier acts and thus promotes the mass transport of the gaseous reactants to the catalyst. It has been shown that it can be advantageous if, in the gas diffusion layer according to the invention, the microporous layer has a property gradient in relation to its base area (in the x, y plane) of the GDL.

The gas diffusion layer according to the invention preferably has a thickness (total thickness of fiber material a) and MPL b)) in the range from 80 to 1000 μm, particularly preferably from 100 to 500 μm. This thickness refers to the uncompressed state of the GDL; H. before installing them in a fuel cell.

Furthermore, the gas diffusion layers preferably have a high overall porosity. This is preferably in the range from 20% to 80%, determined, as described above, by helium density measurement and mercury porosimetry.

Property gradient

As mentioned above, both the flat fiber material a) and the microporous layer b), as well as both, can have at least one property gradient.

The property exhibiting the gradient is in principle selected from the chemical composition of the flat fiber material a) and / or the microporous layer b), the mechanical properties of the flat fiber material a) and / or the microporous layer b), the transport properties of the flat fiber material a) and / or the microporous layer b),

Combinations of these.

The chemical properties of the flat fiber material a) and / or the microporous layer b), which can have a gradient, include, for. B. the content of water repellants, carbon particles, etc. This includes especially the content of PTFE, carbon black, graphite, graphene, carbon nanotubes (CNT),

Carbon nanofibers and mixtures thereof.

The mechanical properties of the sheet-like fiber material a) and / or the microporous layer b), which can have a gradient, include, for. B. the density, the mass per unit area, the porosity and the mean pore diameter.

The density in g / m ³ can be determined by measuring the helium density, as described above.

The mass per unit area in g / m ² can be determined according to ISO 9073-1 or

EN 29073-1: 1992.

The porosity of the GDL can be measured using various known measurement methods, such as B. mercury porosimetry or nitrogen BET method can be determined. To generate a gradient in the mechanical properties, for example, the compression behavior of the microporous layer can be provided with a gradient by varying its composition with respect to at least one of its materials. This also changes the connection to the electrode. Alternatively, a gradient in the mechanical properties can be created by creating a gradient across the width of the material during web consolidation by water jets. This influences mechanical properties and water transport.

The transport properties of the flat fiber material a) and / or the microporous layer b), which can have a gradient, include: the gas permeability of the flat fiber material a) and / or the microporous layer b), the liquid permeability of the flat fiber material a) and / or the microporous layer b), the electrical volume resistance of the gas diffusion layer through the material plane, the thermal volume resistance of the gas diffusion layer through the material plane, the dry diffusion length. The gas permeability perpendicular to the plane of the material can be determined via a Gurley measurement, for which an automated Gurley densometer from Gurley Precision Instruments can be used. During the measurement, the time is determined in seconds until 100 cm ^{3 of} air has flowed vertically through the GDL sample with a sample area of 6.42 cm ² flowing through it at a constant pressure difference. The determination of the Gurley air permeability is described in ISO 5636-5.

The measurement of the gas permeability in l / m ² s can also be carried out according to DIN EN ISO 9237: 1995-12 to determine the air permeability of textile fabrics.

The permeability for liquids, especially for liquid water, perpendicular to the plane of the material (liquid-water permeability "through-plane") can be determined with a so-called "filtration cell" or according to the "Penn State" method [see references ac ]: [a] IS Hussaini and CY Wang, "Measurement of relative permeability of fuel cell diffusion media," Journal of Power Sources, vol. 195, pp. 3830-3840, 2010; [b] J.D. Sole, "Investigation of water transport parameters and processes in the gas diffusion layer of PEMFCs," Virginia Polytechnic Institute, 2008;

[c] J. Benziger, J. Nehlsen, D. Blackwell, T. Brennan, and J. Itescu, "Water flow in the gas diffusion layer of PEM fuel cells," Journal of Membrane Science, vol. 261, pp. 98-106, 2005.

To determine the electrical resistance through the plane (through plane, TP), a method known from the literature with the designation quasi-4-point measuring solution can be used. A separate measurement of the volume resistance of the MPL is not possible with this method.

Two known test methods can be used to determine the thermal resistance through the plane (through plane, TP), the heat flow method or the laser flash method.

The dry diffusion length describes the actual length of the distance that a gas molecule travels through the flat fiber material a) and / or the microporous layer b) in mΐti. It is determined using a stationary Wicke-Kallenbach cell.

In a preferred embodiment, the microporous layer (MPL) has at least one property gradient with regard to at least one chemical and / or physical property. The MPL has the at least one property gradient with respect to its base area, i. H. in plan view or in the x, y plane. If necessary, the MPL can also have a property gradient perpendicular to its base area, i. H. in the direction of the z-axis.

The microporous layer preferably has at least 2, preferably at least 3, in particular at least 4, specifically at least 5, more specifically at least 6 discrete areas which differ in at least one property. In this embodiment, the change in properties between the areas is abrupt. The individual areas can all differ with regard to the same property or (in the case of several different properties) the same properties. This is preferred. However, it is also possible that two or more areas differ with regard to different properties. In a special embodiment, the microporous layer has at least 2, preferably at least 3, in particular at least 4, especially at least 5, more especially at least 6 discrete areas which all differ with regard to one and the same property.

In a special embodiment, each individual area is essentially homogeneous with regard to its properties. In this context, essentially homogeneous is understood to mean that within a range only those property fluctuations occur as they also occur (e.g. due to production) if a gradient is not consciously created.

In an alternative embodiment, the microporous layer has at least one continuous property gradient.

The microporous layer preferably has at least 2, preferably at least 3, in particular at least 4 laterally adjoining strips which differ in at least one property. In a special embodiment, the microporous layer has at least 2, preferably at least 3, in particular at least 4 laterally adjoining strips, which all differ with regard to one and the same property. In particular, each individual strip is essentially homogeneous with regard to its properties.

The property of the microporous layer, which has a gradient, is preferably selected from Gurley gas permeability and dry diffusion length.

Method for producing a gas diffusion layer

Another object of the invention is a method for producing a gas diffusion layer for a fuel cell, which comprises a flat, electrically conductive fiber material a) and a microporous layer b) on one of the surfaces of the fiber material, the microporous layer in relation to the base area of the gas diffusion layer ( in the x, y plane) has at least one property gradient with respect to at least one chemical and / or physical property, in which i) a flat, electrically conductive fiber material a) is provided, ii) the fiber material provided in step i) with a precursor for formation coated a microporous layer, the composition of the precursor being varied during the coating to produce a gradient, iii) subjecting the coated fiber material obtained in step ii) to a thermal aftertreatment.

With regard to the suitable and preferred fiber materials a) used in step i), reference is made in full to the previous statements. The precursors used in step b) preferably contain at least one fluorine-containing polymer, at least one carbon material and, if appropriate, at least one pore former. The fluorine-containing polymers are preferably selected from polytetrafluoroethylene (PTFE) and tetrafluoroethylene-hexafluoropropylene copolymers (FEP). PTFE is preferred. The carbon material is preferably selected from carbon black, graphite, graphene, carbon nanotubes (CNT), carbon nanofibers and mixtures thereof. Carbon black or graphite is preferably used. In a special embodiment, the precursors used in step b) contain at least one pore former. Suitable pore formers are commercially available plastic particles, e.g. B. made of polymethyl methacrylate (PMMA). A suitable particle size is in the range from 10 to 100 mΐti.

The proportion by volume of the pores in the finished microporous layer which can be attributed to the use of a pore former is preferably from 0 to 70% by volume, based on the total volume of the pores in the finished microporous layer.

The fiber material a) is preferably coated with at least 2, preferably at least 3, in particular at least 4 laterally adjoining strips of precursors of different composition to form a microporous layer. Orders from the MPL can be carried out in various ways. While spray, screen printing or Meyer Rod processes are often used in discontinuous production, squeegee, slot nozzle and engraved roller processes are preferred for continuous coating. The MPL layer thickness and the penetration depth can be determined by the Coating process parameters and the viscosity of the coating can be influenced. Finally, there is another thermal treatment, e.g. B. in a drying and sintering furnace. First, drying at a temperature of 100 to 200 ° C and then sintering at a temperature of 300 to 500 ° C can take place.

Fuel cell

Another subject matter of the invention is a fuel cell comprising at least one gas diffusion layer as defined above or obtainable by a method as defined above.

In principle, the gas diffusion layer according to the invention is suitable for all conventional fuel cell types, especially low-temperature proton exchange membrane fuel cells (PEMFC). Reference is made in full to the statements made above on the structure of fuel cells.

It is an advantage of the invention that the transport processes through the gas diffusion layer can be specifically adapted to the gradients of the operating media flowing through the fuel cell and / or the operating parameters of the fuel cell. For this purpose, at least one property gradient of the gas diffusion layer generally corresponds to at least one of the property gradients of the operating media flowing through the fuel cell and / or the operating parameters of the

Fuel cell. FIGURE DESCRIPTION

FIG. 1 shows the top view of a GDL material, the production of which is described in Example 1. On a fiber fleece in DIN A3 format (29.7 x 42 cm, where md denotes the machine direction), 4 laterally adjacent strips with 4 different MPL pastes (paste 1 to 4) each 7 to 8 cm wide were placed in the longitudinal direction applied. Gas diffusion layers in the format 274.8 x 96.5 mm were punched out of the dried and sintered material with the long side transverse to the machine direction. Figure 1 shows 3 alternative punching positions (GDL 1 to 3). The GDL obtained have a property gradient in the direction of the x-axis, GDL 1 having 4 strips each with different properties and GDL 2 and 3 each having 3 strips with different properties.

Figure 2 shows the current density distribution determined according to Example 2 at the cathode of a membrane electrode unit according to the invention with an MPL that has a property gradient (circles) and a membrane electrode unit not according to the invention without property gradients (squares).

The following examples serve to illustrate the invention without restricting it in any way.

EXAMPLES

Example 1 : Production of a gas diffusion layer with a property gradient in the x-direction

From a commercially available electrically conductive fiber fleece with a thickness of 0.145 mm, a weight per unit area of 60 g / m ² and a through plane at 1 MPa compression of 6.6 mOcm ² , sheets in A3 format (29.7 x 42 cm) in the longitudinal direction (machine direction, md) punched out from the GDL roll and coated individually. To produce a microporous layer with a property gradient, 4 laterally adjoining strips with MPL pastes in a width of 7 to 8 cm were applied in the longitudinal direction of the fiber fleece (see FIG. 1). The pastes had a composition according to Table 1. For their production, PTFE, various carbons and plastic particles were dispersed as pore-forming agents in distilled water and applied to the fiber fleece by knife coating. The leaves were then dried at 160.degree. C. and sintered at 400.degree. The resulting MPL loadings were 15 to 22 g / m ^2, depending on the strip.

Table 1

1) each based on the total weight of the paste Gas diffusion layers in the format 274.8 × 96.5 mm were punched out of the sheets obtained with the long side transverse to the machine direction. Figure 1 shows 3 alternative punching positions. The GDL obtained have a property gradient in the direction of the x-axis, GDL 1 having 4 strips each with different properties and GDL 2 and 3 each having 3 strips with different properties.

To produce a fuel cell, as described below, the GDL are installed in such a way that the x-direction (long side) lies in the direction of the direct connection between the supply and discharge of the operating materials to the flow distributor plate. In the case of a flow distributor plate with straight channels, the long side of the GDL is parallel to the gas channels. _{But even with another flowfield design, the supplied O 2} -rich fuel on the cathode side (air side) of the fuel cell first comes into contact with the MPL formed by paste 1 and the discharged 0 _2- poor fuel with the MPL formed by paste 4.

In the case of the GDL 1 according to FIG. 1, the Gurley gas permeability perpendicular to the plane of the material was determined for each of the 4 strips using a Gurley densometer from Gurley Precision Instruments in accordance with ISO 5636-5. The results are shown in Table 2.

In the case of the GDL 1 according to Figure 1, the dry diffusion length was also determined for each of the 4 strips with the aid of a stationary Wicke-Kallenbach cell. The results are also shown in Table 2. Table 2

Example 2:

Measurement of the current density distribution

For an in-situ measurement of the current density distribution, a membrane electrode unit (MEA) according to the invention was used, the cathode of which was based on a gas diffusion layer (GDL), the microporous layer (MPL) of which had a property gradient in the longitudinal direction of the fiber fleece made of 4 laterally adjacent one another Had stripes (GDL 1 from Example 1). A conventional GDL, the MPL of which had no property gradient, was used as the anode. The MPL paste 4 from Example 1 was used to produce it. An MEA was used for comparison, the anode and cathode of which had a GDL not according to the invention (without property gradient) analogous to the anode of the MEA according to the invention.

In the test system, the MEA was supplied with hydrogen and oxygen at a pressure of 1.5 bar each, the relative humidity (RH) of both gases before entry into the measuring arrangement was 95% and the operating temperature of the fuel cell was 74 ° C (coolant temperature on Outlet of the cathode). The excess Hydrogen supply I _A was 1.2 and the amount of excess O2 lo was 2.0. The current density distribution at the cathode was determined at an average value of the tapped current density of 1.8 A / cm ² with a matrix of measuring points, which had 22 measuring locations in the course of the main flow direction of the gas between cathode inlet and cathode outlet and 9 measuring locations perpendicular to the main flow direction of the gas (ie 198 measuring points in total).

Figure 2 shows the current density distribution determined at the cathode of the MEA according to the invention (curve with circular symbols) and not according to the invention (curve with square symbols). For each measuring location 1 to 22 in the main flow direction of the gas, the curves show the mean values over the 9 measuring locations perpendicular to the main flow direction of the gas. It can be seen that the GDL according to the invention has a more uniform current density distribution than the comparison GDL.

Claims

1. Gas diffusion layer for a fuel cell, comprising a) a flat, electrically conductive fiber material and b) a microporous layer on one of the surfaces of the fiber material, the gas diffusion layer with respect to its base area (in the x, y plane) at least one property gradient with respect to at least one has chemical and / or physical property.

2. Gas diffusion layer according to claim 1, wherein the microporous layer has at least one property gradient.

3. Gas diffusion layer according to claim 1 or 2, wherein the microporous layer has a continuous or abrupt property gradient which changes monotonically as a function of the location.

4. Gas diffusion layer according to claim 2 or 3, wherein the microporous layer has at least 2, preferably at least 3, in particular at least 4 regions which differ in at least one property.

5. Gas diffusion layer according to one of claims 2 to 4, wherein the microporous layer has at least 2, preferably at least 3, in particular at least 4 laterally adjoining strips which differ in at least one property.

6. Gas diffusion layer according to claim 5, wherein each individual strip is substantially homogeneous with regard to its properties.

7. Gas diffusion layer according to one of the preceding claims, wherein the property having a gradient is selected from the chemical composition of the flat fiber material a) and / or the microporous layer b), the mechanical properties of the flat fiber material a) and / or the microporous layer b), the transport properties of the flat fiber material a) and / or the microporous layer b),

Combinations of these.

8. A method for producing a gas diffusion layer for a fuel cell, which comprises a flat, electrically conductive fiber material a) and a microporous layer b) on one of the surfaces of the fiber material, the microporous layer in relation to the base area of the gas diffusion layer (in derx, y- Level) has at least one property gradient with respect to at least one chemical and / or physical property, in which i) a flat, electrically conductive fiber material a) is provided, ii) the fiber material provided in step i) is coated with a precursor to form a microporous layer, wherein the composition of the precursor is varied during the coating to produce a gradient, iii) subjecting the coated fiber material obtained in step ii) to a thermal aftertreatment.

9. The method according to claim 8, wherein the composition of the precursor is varied during the coating so that the microporous layer has at least one monotonic property gradient with respect to the base area of the gas diffusion layer (in the x, y plane).

10. The method according to claim 8 or 9, wherein the precursor used in step ii) comprises at least one fluorine-containing polymer, at least one carbon material and optionally at least one pore former.

11. The method according to any one of claims 8 to 10, wherein the fiber material is coated with at least 2 laterally adjoining strips of a precursor to form a microporous layer.

12. A fuel cell comprising at least one gas diffusion layer as defined in one of claims 1 to 7, or obtainable by a method as defined in one of claims 8 to 11.

13. The fuel cell according to claim 12, wherein at least one property gradient of the gas diffusion layer corresponds to at least one of the property gradients of the operating media flowing through the fuel cell and / or the operating parameters of the fuel cell.

14. Use of a gas diffusion layer as defined in one of claims 1 to 7, or obtainable by a method as defined in one of claims 8 to 11, for the production of fuel cells with reduced fluctuations in the current density over the electrode surfaces, especially on the cathode side.